I.A. Environment of Interest
Engineering is based on an understanding of certain physical laws and application of these laws in designing apparatus which will efficiently control the movement of matter and energy. Thermodynamics and heat transfer are two key branches of engineering.
Thermodynamics relates to the relationships between energy and matter, particularly, the relationships among temperature, density, pressure, enthalpy, entropy, etc. Traditionally, thermodynamics has addressed the study of heat engines and refrigeration apparatus.
Heat transfer relates to the modes of heat transfer and methods of predicting heat transfer.
Insulation is a common type of engineering material which is used to control heat transfer and it is often used with a thermodynamic device such as a heat engine or a refrigerator where it is considered to be physically associated with the device, that is, integrated physically to fit and enclose the device. Insulation is not normally integrated into the thermodynamics of the device, that is, made part of the thermodynamic cycle to thereby improve the cycle.
A good thermal insulation system: 1) limits heat transfer by conduction, that is, heat transfer by random molecular motion within a material where the molecules do not move appreciably from a certain point, 2) minimizes heat transfer by convection, that is, physical transport of fluid molecules which carry thermal energy with them from one place to another, and 3) limits thermal radiation.
A good thermal insulation system may actually involve the transfer of significant amounts of heat by controlling where the heat is transferred.
Conduction and convection are normally limited by the use of a barrier layer of material which has a low bulk thermal conductivity. Such material is often porous wherein the cavities in the material are filled with air. Fiberglass, rock wool and asbestos are examples of such material wherein the cavities in the material are interconnected and filled with air (or a specific gas mixture) while wood and styrofoam plastic are examples of materials wherein the cavities in the material are typically filled with air (or a specific gas or gases) and the cavities do not communicate with each other.
Whether the cavities are interconnected or not, heat transfer through these materials by simple conduction through the solid material of the insulation is hindered by the thin direct thermal conduction paths presented by the length of the fibers or by the cavity walls of the material while the fibers or closed cells also inhibit convection of the gas which fills the material. Gases as a class have the lowest thermal conductivities known so that the thermal conduction of the gas in the bulk insulation sets a lower limit to the insulating value of an insulator. An evacuated space may be the basis for still better insulation but such evacuated insulation systems are expensive in design and manufacturing effort.
In many situations, thermal radiation is of limited significance and is considered to be intercepted and controlled by the usual insulation materials.